New palladium(ii) complexes with 3-(2-pyridyl)-5-alkyl-1,2,4-triazole ligands as recyclable C-C coupling catalysts

Article abstract of DOI:10.1039/c9nj02278c

A series of palladium(ii) complexes with 3-(2-pyridyl)-5R-1,2,4-triazoles Pd(L^R)₂ (R = ethyl (4a), n-propyl (4b), i-propyl (4c) and t-butyl (4d)) have been synthesised and characterised by NMR, IR, UV-vis spectroscopy, ESI and MALDI-

Full text of DOI:10.1039/c9nj02278c

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Zakharchenko, D. M. Khomenko, R. O. Doroshchuk, I. V. Raspertova, V. S. Starova, V. V. Trachevskii, S.
Shova, O. V. Severinovskaya, L. M. Martins, . Pombeiro, V. Arion and R. D. Lampeka, New J. Chem., 2019,
DOI: 10.1039/C9NJ02278C.
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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Page 1 of 27
New Journal of Chemistry
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New palladium(II) complexes with 3-(2-pyridyl)-5-alkyl-1,2,4-triazole ligands
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as recyclable C‒C coupling catalysts
DOI: 10.1039/C9NJ02278C
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Borys V. Zakharchenko,a Dmytro M. Khomenko,a,* Roman O. Doroshchuk,a Ilona V. Raspertova,a
Viktoriia S. Starova,a Volodymyr V. Trachevsky,b Sergiu Shova,c Olga V. Severynovska,d Luísa M.
D. R. S. Martins,e,** Armando J. L. Pombeiro,e Vladimir B. Arionf and Rostyslav D. Lampeka a
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a
Department of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street,
64/13, Kyiv 01601, Ukraine
b G.V. Kurdyumov Institute for Metal Physics, N.A.S. of Ukraine, Academician Vernadsky Blvd.36,
Kyiv, 03680, Ukraine
c“Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, 700487 Iasi,
Romania
d
Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 17 General
Naumov Street, Kyiv 03164, Ukraine
eCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco
Pais, 1049-001 Lisboa, Portugal
fUniversity of Vienna, Faculty of Chemistry, Institute of Inorganic Chemistry, Währinger Strasse
42, 1090 Vienna, Austria
* Corresponding author: E-mail: dkhomenko@ukr.net (Dmytro M. Khomenko)
** E-mail: luisammartins@tecnico.ulisboa.pt (Luísa M. D. R. S. Martins)
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Abstract
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A series of palladium(II) complexes with 3-(2-pyridyl)-5R-1,2,4-triazoles Pd(LR)2 (R = ethyl (4a),
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n-propyl (4b), i-propyl (4c) and t-butyl (4d)) has been synthesised and characterDisOeI:d10b.10y39N/CM9NRJ0,22I7R8C,
UV-vis spectroscopy, ESI and MALDI-TOF mass spectrometry. Single-crystal X-ray diffraction
study of 4a-d revealed a square-planar coordination geometry, in which the N^N chelating
monoanionic ligands adopt a trans-configuration around Pd(II) and show notable intraligand C–
H···N hydrogen bonding within the complex. Solid state emission and excitation spectra of 4a-d
showed vibronic structure of the spectra at room temperature. TG/DTA curves of 4a-c revealed that
complexes are stable up to ca. 250 °C, whereas 4d starts to decompose at 230 °C. Compounds 4a-d
efficiently catalyse the nitroaldol (Henry) reaction of benzaldehyde with nitroethane in water,
methanol and ethanol with appreciable diastereoselectivity in favour of the syn-isomer (syn:anti up
to 81 : 19). In addition, 4a-d catalyse the microwave-assisted Suzuki-Miyaura cross-coupling
reaction of bromoanisole with phenyl boronic acid, in the presence of a base. Moreover, these
heterogeneous catalysts can be recovered and reused without loosing activity for several
consecutive cycles.
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Keywords: palladium(II) complexes, 1,2,4-triazole, catalytic activity, Henry reaction, Suzuki-
Miyaura cross-coupling
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New Journal of Chemistry
Introduction
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In recent years, considerable attention has been paid to triazoles, tetrazoles and related N-containing
heterocyclic derivatives. They are versatile ligands for transition metals.1 The chemistry of azoles
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DOI: 10.1039/C9NJ02278C
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and their derivatives is well established. Due to their heterocyclic identity the compounds reveal
different coordination behaviour, as is well-documented, for example, in the case of 1,2,4-triazole-
derived ligands.2 They are considered excellent candidates to build 1D, 2D, and 3D frameworks due
to their peculiar steric profile that is dependent on the presence of substituents, their size and
position in the aromatic ring.3 The ligands are often engaged in hydrogen bonding and have been
used in the preparation of liquid crystals.4,5 Different synthetic routes have been developed over the
last few decades, mainly because some members of the triazole family found application in
pharmacology and agrochemistry.6
In addition, d8 transition metal complexes possess square-planar coordination geometry and form
disk-like molecules packing in stacks, which reveal mesomorphism. Azolate nickel(II),
palladium(II) and platinum(II) complexes were found to exhibit columnar liquid crystal
properties.7,8,9,10,11 In particular, discotic liquid crystals are of great interest due to their high charge-
carrier mobility and the propensity to form one-dimensional aromatic π–π stacks, which can
contribute to their potential applications as field-effect transistors,12 photovoltaic solar cells, optical
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data storage devices,13,14 sensors and electroluminescent displays.15,16
Palladium(II) and
platinum(II) complexes with 1,2,4-triazoles are strongly emissive both in solution and in the solid
state, and have recently received much attention17,18,19 in view of their use in light-emitting
devices20,21,22 and as potential luminescent probes for biological and medical purposes.23,24
Moreover, promising catalytic activity in C–C cross coupling reactions is well-documented as well.
Among the wide range of this type of reactions, Suzuki – Miyaura and Henry reactions occupy a
special place, since they are the examples of simple and convenient ways to form a C–C bond.25,26,27
These reactions are carried out using a variety of palladium complexes. The successful use of
certain palladium complexes as catalysts depends on the nature of the ligand, namely, on its sterical
and donor properties. In addition, considerable attention has been paid to N,N-chelating
ligands.28,29,30,31
Recently we reported on the synthesis and characterisation of palladium(II) complexes with a large
number of triazole-derived ligands.32,33,34,35 In particular, we discovered that 3-(2-pyridyl)-1,2,4-
triazoles are able to form mono- and tetranuclear complexes of the type Pd(HL)Cl2, PdL2 and
Pd4L4Cl4.
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Herewith we describe the synthesis and characterisation of several closely related palladium(II)
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complexes of the type Pd(LR)2, where R = ethyl (4a), n-propyl (4b), i-propyl (4c), t-butyl (4d)
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DOI: 10.1039/C9NJ02278C
(Scheme 1). Catalytic activity of 4a-d towards efficient C‒C cross coupling reactions (Suzuki-
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Miyaura and Henry) is reported as well. These reactions are frequently used in key steps of eco-
benign synthetic methodologies for fine chemical, pharmaceutical, agrochemical and modern-
material industries.36,37,38
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Results and Discussion
Synthesis and characterization of proligands and palladium(II) complexes. The chelating
triazole proligands (3a-d) were prepared in two steps involving acylation of the hydrazides (2a-d)
with picolinic iminoester formed in situ from the reaction with CH3ONa, followed by reflux of a
methanolic solution of the generated amidrazone (Scheme 1). The palladium(II) complexes (4a-d)
were prepared by reacting triazoles (3a-d) with PdCl2(CH3CN)2 in acetonitrile at room temperature
(Scheme 1).
O
R
R
N
H
N
H
N
N
N
N
N
H2N
PdCl2(CH3CN)2
CH3CN, Et3N
2a-d
Pd
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CH3ONa, reflux
N
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N
3a-d
N
N
ethyl (a) n-propyl(b) i-propyl (c) t-butyl (d)
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R
4a-d
Scheme 1. Synthesis of proligands and palladium(II) complexes.
IR spectra. The IR spectra of 3a-d reveal common features due to their structural similarity,
namely the presence of triazol-pyridyl moiety in all molecules (see ESI, Figs S1-2†). Characteristic
bands ν(N–H)HTrz at 3170–3050 cm‒1 and ν(C=N)Py,Trz at 1595–1420 cm‒1 were observed. The IR
spectra of 4a-d (see ESI, Figs S3-4†) show characteristic absorption bands of the ligands, which are
slightly shifted relative to metal-free triazoles and characterised by the absence of the absorption
bands of stretching vibrations ν(N–H). This is caused by deprotonation of triazole. Of note is also
the shift of bands ν(C=N)Py,Trz (Δν = 15‒20 cm‒1) upon coordination.
MALDI-TOF and ESI mass spectra. To prove the formation of 4a-d MALDI-TOF and ESI mass
spectra were measured. MALDI-TOF mass spectra were recorded in linear and reflectron modes for
positive and negative ions. Positive ion MALDI-TOF spectra showed a number of signals of low
intensity attributed to products of fragmentation of molecular ions. Of note is the tendency to form
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New Journal of Chemistry
associates, which increases along with branching of alkyl chain in the proligand and is most typical
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for 4c and 4d. The presence of strong peaks with m/z attributed to [M–H]– was observed in the
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DOI: 10.1039/C9NJ02278C
negative ion mass spectra. Unfortunately, low-resolution of the obtained patterns has to be
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mentioned (see ESI, Figs S5-6†). Positive ESI mass spectra revealed strong peaks attributed to
[Pd(LR)2+H]+ with isotopic patterns matching well the theoretically calculated (see ESI, Figs S7-
8†). In addition, signals for proligands at m/z 175, 189 and 202.9 corresponding to [HL+H]+ were
also observed for all Pd(II) complexes.
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1H and 13C NMR, UV-vis absorption and emission spectra. Investigation of proligands 3a-d (see
ESI, Figs S9-12†) and palladium(II) complexes 4a-d (see ESI, Figs S13-16†) in solution was
performed by NMR spectroscopy and UV-vis spectrophotometry. The ortho-pyridine proton in 4a-d
showed a resonance doublet signal (J = 5.0–5.5 Hz) in the region 10.03–10.16 ppm, i.e. this proton
becomes less shielded compared to that in the proligands. The Δδ for ortho-pyridine proton is in the
region of +1.38 to +1.42 ppm. The origin of this shift becomes clear from the inspection of the X-
ray diffraction structures for 4a-d. The ortho-Py proton is involved in the formation of
intramolecular hydrogen bonding (C–H···N) with the triazole ring of adjacent ligand. This is also in
accord with previously reported data for analogous compounds.39 Changes in the chemical shifts of
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other pyridine protons in H NMR spectra of 4a-d relatively to those of the corresponding
proligands are similar.
Significant broadening of signals corresponding to triazole ring carbon atoms in 13С NMR spectra is
presumably due to the existence of several tautomeric forms for 3a-d in solution.32 This hypothesis
was supported by the appearance of sharp carbon resonances in the spectra of 4a-d with
monoanionic ligands.
The UV-vis absorption spectra of Pd(II) complexes in CHCl3 are shown in Figure 1. These spectra
are characterised by three absorption bands. The most intense absorption is observed at 245 nm
arising from π-π* transition in pyridine cycle, the second at ca. 271 nm due to π-π* transitions in
triazole ring and the third one at ca. 350 nm due to ligand to metal charge transfer (LMCT).
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Figure 1. Absorption spectra of PdL2 in CHCl3. CPdL2= 1∙10-5М.
The molar extinction coefficients of bands at 245 and 350 nm follow the order 4a > 4c > 4d (Fig. 1)
in accord with the decrease of degree of solvation of 4a-d. In this small series polarisation of ligand
molecules and steric volume of the lateral chain in the ligand increase (steric constant values Es for
ethyl, i-propyl and t-butyl are of ‒0.07, ‒0.47 and ‒1.54, respectively)40 and as result solvation of
complexes in chloroform decreases. In addition, attachment of t-butyl group to the triazole ring of
the ligand leads to a minor hypsochromic shift of LMCT band of 4d relative to that of 4a (Δλ ca. 4
nm) and redistribution of the absorption intensity between the maxima at 245 and 271 nm. This
shift can be also caused by appearance of an excess negative charge on conjugation chain due to
hyperconjugation effect of t-butyl group. Interestingly, the presence of an unbranched aliphatic
chain in the structure of the molecule improves the solvation of complex molecules and causes a
significant increase in intensity of the absorption bands. Thus, the absorbance of 4b (n-propyl Es =–
0.36) is approximately by factor 1.5 higher than the absorbance of other complexes. The values of
molar absorbtivities for 4a-d in CHCl3 are shown in Table 1.
Table 1. Molar absorptivities of 4a-d in CHCl3.
Substance
λabs1
,
ε1, L·mol-1·cm-1 λabs2, nm ε2, L·mol-1·cm-1
λabs3
nm
,
ε3, L·mol-
1·cm-1
nm
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245
244
245
2,75·104
3,49·104
2,71·104
2,57·104
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271
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273
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3,16·104
2,06·104
2,31·104
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349
8,19·103
1,02·104
6,38·103
6,44·103
4a
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Crystal structures. The solid state structures for 4a-d were determined by single-crystal X-ray
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diffraction analysis. Compounds 4a and 4b are similar and only the crystal structure of the n-propyl
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derivative 4b, is described here in detail. The results of this study for 4b are illusDtrOaI:t1e0d.10i3n9/CF9iNgJu0r2e2782C,
while the bond lengths and angles are quoted in Tables S1 and S2. 4b crystallizes in the triclinic
space group P-1. The crystal structure consists of molecular complexes, as shown in Figure 2 (left).
No co-crystallised solvent has been found in the crystal. The central Pd atom occupies a special
position that resides on a crystallographic inversion centre. The complex exhibits a square-planar N4
coordination geometry provided by two symmetrically equivalent bidentate ligands with Pd1–N1
and Pd1–N2 distances of 2.039(2) Å and 1.995(2) Å, respectively. It is of note, that the planar
configuration is stabilised by two intramolecular C‒H···N hydrogen bonds (C1‒H 0.93 Å, H···N3(1
– x, ‒y, ‒z) 2.35 Å, C1···N3 3.159(3) Å, C1‒H···N3 145.1). The crystal structure of 4b can be
described as a two-dimensional supramolecular architecture which grows in ab plane through the
short Pd1···N4 contacts of 3.319(2) Å at apical positions with two adjacent molecules, as shown in
Figure 2 (right). As a result, the environment of Pd atom can be described as a tetragonally distorted
4+2 coordination. The shortest intermolecular metal-metal separation within 2D network is equal to
5.434(1) Å. The formation of a supramolecular layer is additionally sustained by the C4‒H···N3
intermolecular hydrogen bonding (C4‒H∙∙∙N4 [C4‒H 0.93 Å, H···N4(2 – x,1 –y, –z) 2.75 Å,
C4···N4 3.473(3) Å, C4‒H···N4 135.4).
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Figure 2. Left: Molecular structure of 4b with atom labelling scheme and thermal ellipsoids at 50%
probability level. Right: View of 2D supramolecular network in the crystal structure of 4b (n-propyl
fragments are omitted for clarity).
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A similar molecular complex [PdL2] with square-planar N4 coordination geometry has been found
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in the crystal structure of 4c (Fig. 3, left). Compound 4c crystallizes in the monoclinic P21/c space
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group. Pd atom occupies a special position which resides on the inversion centreDbOeI:in10g.10c3o9/oCr9dNJin02a2t7e8dC
by two symmetry equivalent bidentate ligands in square-planar geometry. Pd atom in 4c
additionally interacts with two N3 atoms of two adjacent molecules at Pd1–N3(x – 1, y, z) at a
distance of 3.416(2) Å to form infinite chain running along 100 direction. A view of a part of one-
dimensional {PdL2}n array is shown in Figure 3, right.
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Figure 3. Left: Molecular structure of 4c with atom labelling scheme and thermal ellipsoids at 50%
probability level. Right: 1D {PdL2}n chain showing 4+2 coordination environment of Pd atom. H-
bond parameters: C1–H···N3 [ C1–H 0.93 Å, H···N3 2.31 Å, C1···N3(1 – x, 2 – y, 1 – z) 3.13(1) Å,
C1HN3 145.5.]
Compound 4d crystallizes in the monoclinic space group C2/m. The crystal structure consists of
neutral complexes [PdL2] and CHCl3 and H2O as co-crystallised solvent molecules in 3:2:2 ratio.
The asymmetric part of the unit cell (Fig. 4, left) contains two crystallographically independent, but
chemically identical complexes (designated A and B). Both molecules have own symmetry with a
special position on the crystallographic reflection plane for A and Pd atom on the inversion centre
for molecule B. The Pd atoms exhibit the same square-planar N4 coordination geometry with the
Pd–N distances in the range between 1.990(7) and 2.029(7) Å (for both A and B molecules), that
are very close to those observed for 4b. The analysis of the crystal structure revealed two-
dimensional supramolecular networks (Fig. 4, right) parallel to 101 plane, which are formed
through the O‒H∙∙∙N hydrogen bonding between water molecules as proton donors and the nitrogen
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atoms of the heterocycles as proton acceptors, as well as via - stacking interactions between
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aromatic rings of the adjacent centrosymmetrically related complexes.
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Figure 4. Left: Extended asymmetric unit of 4d with atom labelling and thermal ellipsoids at 50%
probability level. Atoms obtained by symmetry-transformations are shown transparent. Right: View
of 2D supramolecular network. H-bonds and centroid-to-centroid distances of 3.800 and 3.890 Å
are drawn in dashed black and brown lines, respectively.
Solid state emission spectra. Complexes 4a-d do not exhibit fluorescence in chloroform at
concentrations less than 1×10‒5 М. The emission spectra could not be measured at higher
concentrations of Pd(LR)2 in CHCl3 because of their limited solubility. Therefore, emission and
excitation spectra were recorded for the Pd(II) complexes 4a-d in solid state. A vibronic structure of
the spectra at room temperature is of note (Fig. 5).
Excitation spectra were fitted to a sum of three individual Gaussian components and characterised
by three bands with maxima at 240, 275 and 345 nm (Fig. 5, left). The intensities of these bands are
less pronounced than those of corresponding bands in the optical spectra. In the absence of the
solvation effects excitation intensity decreases in the order: 4a > 4b > 4c, in line with the increase
of Es values and decrease of +M effect of alkyl substituent in the ligand structure.
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Figure 5. Excitation (left) and fluorescence (right) spectra of Pd(LR)2 in solid state at room
temperature.
Gaussian band analysis of emission spectra also allowed for the identification of three types of
electron transitions: π-π* in pyridine ring responsible for complex fluorescence at 440 nm, π-π*
in triazole ring that give the most intense emission band at 480 nm and LMCT emittion in green
spectral range with maximum at ~520 nm (Fig. 5, right). Pd(Ln-Pr)2 has more intense
fluorescence in a long wave region 510-560 nm compared to complexes 4a, c-d. Vibronic
structure of emission and excitation spectra of 4a-d (Fig. 5) is presumably due to strong
localisation of the complex molecules in the crystal lattice and similar internuclear distances in
the ground and excited states of the molecule. Emission spectrum of 4d shows a slight
hypsochromic shift Δλ ≈ 10 nm relative to the spectra of 4a-c and has the most pronounced
vibronic structure (Fig. 5, right). The hypsochromic shift is likely due to the presence of highly
symmetrical t-butyl groups in the complex. In turn, this leads to inhibition of low frequency out-
of-plane motions associated with Stoke`s shift.41 Pronounced vibronic structure is likely related to
highly ordered voids formed in 4d after drying, which results in a highly structured emission at
room temperature as the exciton couples to vibrations in the lattice.
TG/DTA measurements. Thermogravimetric analysis (TG/DTA) of the palladium(II)
complexes 4a-d (Fig. S17†) was performed to get information about their thermal stability and a
general pathway of thermal decomposition. Under air the complexes 4a-c are stable up to ca. 250
°C. The thermal decomposition of the complexes 4a-c occurs in two main steps. The first mass
loss at 300–350 °C undergoes without significant thermal effect and the second one at 420‒430
°C is an exothermic process. The complex 4d is completely decomposed at about 230 °C.
10

Page 11 of 27
New Journal of Chemistry
1
2
3
4
5
Catalytic activity. The catalytic activity of 4a-d was tested in the microwave-assisted Suzuki-
View Article Online
DOI: 10.1039/C9NJ02278C
Miyaura (Scheme 2) and the Henry (Scheme 3) C–C coupling reactions.
6
7
8
9
Br
B(OH)2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4 a-d
+
OMe
base, MW
OMe
Scheme 2. Microwave (MW) assisted Suzuki-Miyaura C–C cross-coupling reaction of 4-
methoxybromobenzene with phenyl boronic acid, carried out by employing 4a-d as catalysts.
CHO
Ph
Ph
O
O
4 a-d
r.t.
+
+
NO2
NO2
O2N
Scheme 3. Henry C–C coupling reaction of benzaldehyde with nitroethane, carried out by
employing 4a-d as catalysts.
As a model system for the most widely-applied transition metal catalysed carbon–carbon bond
forming reaction,34,42 the Suzuki-Miyaura cross-coupling, the commonly used reaction of 4-
methoxybromobenzene with phenylboronic acid, accomplishing 4-methoxybiphenyl, was chosen
(Scheme 2). Microwave (MW) irradiation heating which constitutes an appealing alternative energy
source43,44 (generating heat evenly throughout the reactor) enhancing product purity, selectivity, and
yield as well as being more energy efficient and economical in comparison to conventional heating
methods was used. Several solvents (e.g., water, methanol, ethanol, chloroform, acetonitrile,
toluene, hexane) were tested in order to find the most suitable one for the MW promoted reaction. It
was found that a solvent with a high ability to convert electromagnetic energy into heat, i.e., high
loss factor tan , was required, thus discarding the possibility of using solvents with low loss
tangent values such as hydrocarbons (tan  = 0.040 for toluene or tan  = 0.020 for hexane45).
Catalytic results for ethanol (tan  = 0.941, high), water (tan  = 0.123, medium) and chloroform
45
(tan  = 0.091, low) are presented in Table 2. Optimisation of the reaction conditions led to the
use of ethanol and potassium carbonate as the most appropriate solvent or base, respectively, for the
catalytic runs (Table 2). The catalytic activity of 4a-d was tested at 50 to 100 ºC, during 15 to 90
min.
11

New Journal of Chemistry
Page 12 of 27
1
2
3
4
5
Table 2. Selected dataa for the MW-assisted Suzuki-Miyaura reaction between 4-
View Article Online
DOI: 10.1039/C9NJ02278C
methoxybromobenzene and phenyl boronic acid catalysed by 4a-d.
6
7
8
9
Entry Catalyst
Solvent Base
K2CO3
Yield /%b TONc x 10-2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2
3
61
49
27
25
91
40(*)
75
63
51
39
24
92
45(*)
79
52
47
31
23
86
6.1
4.9
2.7
2.5
9.1
4.0
7.5
6.3
5.1
3.9
2.4
9.2
4.5
7.9
5.2
4.7
3.1
2.3
8.6
H2O
Cs2CO3
K2CO3
Cs2CO3
K2CO3
CHCl3
4
4a
5
6
EtOH
7
Cs2CO3
K2CO3
Cs2CO3
K2CO3
Cs2CO3
K2CO3
8
H2O
9
10
11
12
13
14
15
16
17
18
19
CHCl3
4b
EtOH
H2O
Cs2CO3
K2CO3
Cs2CO3
K2CO3
Cs2CO3
K2CO3
4c
CHCl3
EtOH
12

Page 13 of 27
New Journal of Chemistry
20
21
22
23
24
25
26
27
28
29
30
31
38(*)
71
49
36
40
32
82
31(*)
67
29
11
6
3.8
7.9
4.9
3.6
4.0
3.2
8.2
3.1
6.7
2.9
1.1
-
1
2
3
4
5
6
7
8
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DOI: 10.1039/C9NJ02278C
Cs2CO3
K2CO3
H2O
Cs2CO3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
K2CO3
CHCl3
Cs2CO3
4d
K2CO3
EtOH
Cs2CO3
cis-[PdCl2(PPh3)2] EtOH
K2CO3
K2CO3
K2CO3
PdCl2
-
EtOH
EtOH
aReaction conditions: 80 ºC, MW irradiation (10 W power), 1.5 h, catalyst (1 mol), 4-
methoxybromobenzene (1.0 mmol), phenyl boronic acid (1.2 mmol), base (1.5 mmol) and solvent
b
(3.0 mL). Number of moles of 4-methoxybiphenyl per 100 mol of 4-methoxybromobenzene
(calculated by H NMR). Number of moles of 4-methoxybiphenyl per mole of catalyst. (*) under
1
c
conventional heating instead of MW irradiation.
After 1.5 h of MW irradiation under the optimised conditions (e.g., entry 18 of Table 2 for 4b) the
conversion of 4-methoxybromobenzene into methoxybiphenyl is essentially complete (yield 92%).
In the tested conditions, compounds 4a-d exhibited much superior catalytic activity than the
commonly used cis-[PdCl2(PPh3)2] (compare entries 5, 12, 19 or 26 and 29 of Table 1) or the
chloride salt PdCl2 (compare entries 5, 12, 19 or 26 and 30 of Table 1), being very promising
catalysts under mild (80 ºC, 1.5 h in air) and environmentally benign (solvent and heating method)
conditions for the fast Suzuki-Miyaura cross coupling.
Moreover, all compounds 4a-d were found to be stable in the reaction medium. Their possible
reduction to Pd(0) nanoparticles46 upon the C–C coupling reaction was discarded by UV-vis
analysis after the reaction. In fact, the characteristic plasmonic bands47 of the Pd nanoparticles were
13

New Journal of Chemistry
Page 14 of 27
not detected. In addition, the mercury drop experiment48 ,49(see Experimental section) led to a
1
2
3
4
5
negligible difference in the catalytic performance in the presence of Hg(0), also suggesting that
View Article Online
DOI: 10.1039/C9NJ02278C
heterogeneous Pd(0) is not involved in the 4a-d catalysed Suzuki C–C coupling. As an example,
6
7
8
9
after the first catalytic run, 4b was separated upon centrifugation, washed and dried and no visible
decomposition was detected. Then, it was recycled for five consecutive cycles maintaining constant
reactivity as depicted in Figure 6.
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Effect of the number of consecutive cycles on the yield of the 4-methoxybiphenyl
obtained by MW-assisted Suzuki-Miyaura cross coupling of 4-methoxybromobenzene and phenyl
boronic acid catalysed by 4b.
The used low power (10 W) MW irradiation provides a much more efficient synthetic method than
conventional heating (non-pressurised refluxing, see Experimental Section), allowing for the
attainment of significantly better yields in much shorter times (compare e.g., entries 5 and 6 of
Table 2 for 4a). The promoting effect of MW methods was observed in other cases.50,51,52
As a model system for the nitroaldol (Henry) Csp3–Csp2 coupling reaction between a nitroalkane
and a carbonyl compound leading to -nitroalcohols, the reaction of benzaldehyde with nitroethane
was chosen (Scheme 3). The catalytic activity of 4a-d was tested at room temperature to 75 ºC, in
water and alcohols during 24 h. The products of Henry reaction are a mixture of the β-nitroethanol
diastereoisomers (syn and anti forms, with a general predominance of the former, Table 3),
1
according to H NMR analysis (see experimental part). No significant nitroaldol reaction between
benzaldehyde and nitroethane was observed in the absence of 4a-d, even in the presence of
palladium chloride (Table 3).
14

Page 15 of 27
New Journal of Chemistry
Table 3. Selected optimised dataa for the Henry reaction between benzaldehyde and nitroethane
1
2
3
4
5
6
7
8
9
catalysed by 4a-d.
View Article Online
DOI: 10.1039/C9NJ02278C
Entry Catalyst
Solvent Temperature Yield
/ºC
Selectivity TONd
(syn/anti)c
/%b
82
76
70
64
73
66
84
81
69
62
79
74
88
81
76
70
83
80
92
88
81
78
79
73
45
9
1.6  102
1
2
3
H2O
25
75
25
75
25
75
25
75
25
75
25
75
25
75
25
75
25
75
25
75
25
75
25
75
25
25
25
80:20
1.5  102
1.4  102
1.3  102
1.5  102
1.3  102
1.7  102
1.6  102
1.4  102
1.2  102
1.6  102
1.5  102
1.8  102
1.6  102
1.5  102
1.5  102
1.7  102
1.6  102
1.8  102
1.8  102
1.6  102
1.6  102
1.6  102
1.5  102
9  101
77:23
77:23
69:31
75:25
68:22
81:19
79:21
76:24
73:27
77:23
69:31
79:21
76:24
77:23
68:32
76:24
70:30
79:21
73:27
76:24
69:31
74:26
70:30
72:28
71:29
-
10
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12
13
14
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17
18
19
20
21
22
23
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26
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31
32
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45
46
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57
58
59
60
EtOH
MeOH
H2O
4a
4
5
6
7
4b
4c
4d
8
9
EtOH
MeOH
H2O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
EtOH
MeOH
H2O
EtOH
MeOH
cis-[PdCl2(PPh3)2] H2O
1.8  101
PdCl2
-
H2O
H2O
-
-
15

New Journal of Chemistry
Page 16 of 27
aReaction conditions: catalyst (5 mol), benzaldehyde (1 mmol), nitroethane (4 mmol) and solvent
1
2
3
4
(2 mL) in air, 24 h. bNumber of moles of β-nitroethanol per 100 mol of benzaldehyde. cDetermined
View Article Online
DOI: 10.1039/C9NJ02278C
by 1H NMR. dNumber of moles of β-nitroethanol per mole of catalyst.
5
6
7
8
9
Compounds 4a-d exhibit appreciable diastereoselectivity (up to syn:anti of 81:19, Table 2) in a
reaction where the stereochemical control of the carbon centres is rather difficult due to the
epimerisation of the nitro moiety.53,54 Moreover, they present good catalytic activity even at room
temperature (Table 3). In fact, an increase of the reaction temperature does not improve the yield or
the selectivity of the reaction (Table 3) regardless the solvent used. It is worth noting that water
revealed to be the most suitable solvent in the tested conditions, allowing higher yields and a better
stereochemical control (Table 3), another advantage of 4a-d towards the development of sustainable
catalytic systems for this C–C coupling reaction.
The reaction mechanism is expected55,56,57 to involve metal-assisted (upon coordination)
deprotonation of the methylene group of nitroethane (to give a nitronate species), followed by
activation of benzaldehyde for its electrophilic attack (with C–C coupling) toward the nitronate.
Hence, the Pd(II) complexes 4a-d appear to be able to act as Lewis acids for the above activations.
The hypothesis of leaching under the tested conditions was verified for the best diastereoselectivity
assay: no nitroethanol was formed in an assay performed with the solution recovered from a first
run after centrifugation and filtration of the supernatant solution to which new portions of all
reagents except 4b were added. The catalyst recovered by centrifugation from the first run was
tested in subsequent runs by addition of fresh reagents. The recyclability of the system is depicted
in Figure 7: it maintains almost the original level of activity after several reaction cycles (in a
second, third, fourth and fifth run, the observed activities were 99.4, 98.7, 97.7 and 95.9% of the
initial one, respectively) to the β-nitroethanol diastereoisomers.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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38
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. Effect of the number of consecutive cycles on the yield of the β-nitroethanol
diastereoisomers formed in reaction of benzaldehyde with nitroethane catalysed by 4b.
16

Page 17 of 27
New Journal of Chemistry
1
2
3
4
5
Generally, activity of the investigated compounds is almost identical in both reactions and yields of
View Article Online
DOI: 10.1039/C9NJ02278C
title compounds vary in 15% range, taking into account the permanence of other conditions (Figs
6
7
8
9
S18-S23). In most cases there is no strong dependence of reaction yields from the volume of non-
coordinating substituent in proligands, but in a few cases such kind of tendency is observed. For
example, Henry reaction performed in water solution at 25 ºC. In this case, an increase of yields
together with volume of substituent from ethyl to t-butyl was clearly observed (Figs 8, S18). This
trend persists also in methanol solution but here maximum yield of the reaction was observed when
4c was used as catalyst (Fig. S20). In the case of Suzuki-Miyaura reaction there is another tendency
– the yields of the title compounds in reactions catalysed by 4d are the lowest among the
investigated compounds (Figs S21-S23). It could be concluded that catalytic activity of investigated
compounds is slightly changing upon varying of substituents in pyridyl-triazole ring, but such minor
changes are also very important in industrial chemistry.58 Even though strong dependences of
catalytic activity on the structure were not observed, it was clearly shown that the title compounds
are efficient recyclable catalysts of Suzuki-Miyaura and Henry reactions.
10
11
12
13
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20
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22
23
24
25
26
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29
30
31
32
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34
35
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37
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 8. Yields of the Henry reaction provided in water at 25 ºC and catalysed by 4a-d.
Conclusion
In summary, a new series of alkyl pyridyl-triazole proligands HLR (3a-d) and their corresponding
palladium(II) complexes Pd(LR)2 (4a-d) were prepared and characterised. X-ray diffraction
analyses revealed that all complexes have the same molecular structure and exhibit a square-planar
N4 coordination geometry provided by two symmetrically equivalent bidentate ligands. Such kind
of coordination mode makes possible the formation of two intramolecular hydrogen bonds between
adjacent ligands, which is a very important feature in the context of proton conduction in liquid
crystals. Thus 4a-d could be as useful models for further assembly of metallomesogens, in which
the liquid-crystal properties, such as the stability of the mesogenic phases and photophysical
17

New Journal of Chemistry
Page 18 of 27
behaviour can be fine-tuned by varying the alkyl chain length. In contrast to the molecular structure,
1
2
3
crystal packing of title compounds depends on branching of alkyl chain in ligands and in the case of
View Article Online
4d, which contains the bulkiest t-butyl substituent, formation of voids filled withDsOoIl:v10e.1n0t39m/Co9NleJ0c2u2l7e8Cs
is observed. From our point of view this structural feature could explain the most pronounced
vibronic structure of solid-state emission and excitation spectra of this compound when compared
to 4a-c. Moreover, compounds 4a-d are very promising recyclable catalysts for the C-C cross
Suzuki-Miyaura and Henry coupling reactions, and a step towards the design of sustainable
catalytic processes for these important industrial transformations.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
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35
36
37
38
39
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Conflicts of interests
There are no conflicts of interest to declare.
Acknowledgements.
This work was supported by the Romanian National Authority for scientific Research and
Innovation, POCPOLIG, grant no.67/08.09.2016. It has been partially supported by the Fundação
para a Ciência e a Tecnologia (FCT), Portugal, through the research projects PTDC/QUI-
QUI/109846/2009, PTDC/QEQ-ERQ/1648/2014 and UID/QUI/0100/2019.
EXPERIMENTAL
Starting Materials. All reagents were obtained commercially unless otherwise noted and used as
received. Respective hydrazides 2 were prepared by standard procedures 32. cis-PdCl2(CH3CN)2 was
purchased from Aldrich and used as received. All solvents used were laboratory reagent grade.
Typical procedure for preparation of proligands (3a-d). Sodium methoxide methanol solution (5
ml, 35 wt %) was added to a solution of 2-cyanopyridine (0.05 mol) in methanol (20 ml) and stirred
at room temperature for 1 h. Then a hydrazide was added to solution of generated iminoester and
mixture was refluxed for 10 h. After cooling to room temperature the solvent was removed in
vacuo. The resulting suspension was diluted with water and acidified with acetic acid (2 ml) to
produce a white solid. The crystals were separated by filtration, dried and recrystallised from
toluene.
3-(2-Pyridyl)-5-ethyl-1,2,4-triazole (3a). Yield: 4.55 g (74%), white crystals. m.p.: 165-166 °C.
3
1H NMR (400 MHz, CDCl3) δ: 12.98 (br s, 1H, NH), 8.75 (d, JH,H = 4.3 Hz, 1H, Py-H6), 8.21 (d,
3
3JH,H = 7.5 Hz, 1H, Py-H3), 7.85 (t, 3JH,H = 7.6 Hz, 1H, Py-H4), 7.38 (t, JH,H = 6.4 Hz, 1H, Py-H5),
2.89 (q, 3JH,H = 7.6 Hz, 2H, H9), 1.40 (t, 3JH,H = 7.5 Hz, 3H, H10) ppm. 13C NMR (101 MHz, CDCl3)
18

Page 19 of 27
New Journal of Chemistry
δ: 165.88 (C7), 155.95 (C8), 149.55 (C6), 147.28 (C2), 137.67 (C4), 124.75 (C3), 121.93 (C5), 21.59
1
2
3
4
5
6
7
8
(C9), 12.80 (C10) ppm. IR (KBr, cm-1): 3132(w), 2976 (m), 2738 (m), 1807(w), 1594(s), 1448(s),
View Article Online
1348(s), 1136(w), 1060(m), 916(w), 735(m). LC/MS: m/z 175 [HLEt +H]+. EDleOmI: 1e0n.1t0a39l/Ca9nNaJ0ly22s7i8sC:
Anal. Calcd for C9H10N4 (Mr=174.09), %: C, 62.05; H, 5.79; N, 32.16. Found, %: C, 61.83; H,
5.72; N, 31.91.
9
3-(2-Pyridyl)-5-n-propyl-1,2,4-triazole (3b). Yield: 4.59 g (61%), white crystals. m.p.: 168-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
3
169 °C. H NMR (400 MHz, CDCl3) δ: 13.24 (br s, 1H, NH), 8.76 (d, JH,H = 4.1 Hz, 1H, Py-H6),
8.21 (d, 3JH,H = 7.5 Hz, 1H, Py-H3), 7.84 (t, JH,H = 7.6 Hz, 1H, Py-H4), 7.38 (t, JH,H= 7.3 Hz, 1H,
Py-H5), 2.83 (t, 3JH,H = 7.6 Hz, 1H, H9), 1.86 (sext, 3JH,H = 7.4 Hz, 2H, H10), 1.01 (t, J = 7.3 Hz, 3H,
H11) ppm. 13C NMR (101 MHz, CDCl3) δ: 164.64 (C7), 155.43 (C8), 149.53 (C6), 147.24 (C2),
137.65 (C4), 124.69 (C3), 121.95 (C5), 30.05 (C9), 21.90 (C10), 13.95 (C11) ppm. IR (KBr, cm-1):
3024(w), 2959 (m), 1761 (w), 1591 (s), 1568 (w), 1455 (s), 1277 (w), 1125 (w), 1045 (m), 996 (m),
893 (m), 730 (s), 613 (w), 492 (w), 403 (w). LC/MS: m/z 189 [HLn-Pr+H]+. Elemental analysis:
Anal. Calcd for C10H12N4 (Mr=188.22), %: C, 63.81; H, 6.43; N, 29.77. Found, %: C, 63.58; H,
6.72; N, 29.96.
3
3
3-(2-Pyridyl)-5-i-propyl-1,2,4-triazole (3c). Yield: 4.62 g (67%), white crystals. m.p. :145-
1
3
146 °C. H NMR (400 MHz, CDCl3) δ: 12.70 (br s, 1H, NH), 8.73 (d, JH,H = 4.1 Hz, 1H, Py-H6),
8.21 (d, 3JH,H = 7.9 Hz, 1H, Py-H3), 7.84 (t, 3JH,H = 7.3 Hz, 1H, Py-H3), 7.37 (t, 3JH,H = 6.8 Hz, 1H,
Py-H5), 3.20 (spt, JH,H = 6.7 Hz, 1H, H9), 1.42 (d, JH,H = 7.0 Hz, 6H) ppm. C NMR (101 MHz,
CDCl3) δ: 170.00 (C7), 155.28 (C8), 149.57 (C6), 147.29 (C2), 137.60 (C4), 124.73 (C3), 121.97
(C5), 28.28 (C9), 21.76 (C11and C10) ppm. IR (KBr, cm-1): 3425 (w), 3125 (m), 2976 (s), 2760 (m),
1597 (s), 1570 (w), 1453 (s), 1336 (w), 1027 (s), 1096 (m), 807 (m), 743 (s), 625 (w). LC/MS: m/z
189 [HLiso-Pr+H]+. Elemental analysis: Anal. Calcd for C10H12N4 (Mr=188.22), %: C, 63.81; H,
6.43; N, 29.77. Found, %: C, 63.84; H, 6.37; N, 29.61.
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3-(2-Pyridyl)-5-t-butyl-1,2,4-triazole (3d). Yield: 4.78 g (69%), white crystals. m.p.: 173-174 °C.
1H NMR (400 MHz, CDCl3) δ: 12.69 (br s, 1H, NH), 8.74 (d, JH,H = 4.4 Hz, 1H, Py-H6), 8.22 (d,
3
3JH,H = 7.8 Hz, 1H, Py-H3), 7.82 (t, JH,H = 7.6 Hz, 1H, Py-H4), 7.35 (t, JH,H= 7 Hz, 1H, Py-H5),
1.46 (s, 9H, H10-12) ppm. 13C NMR (101 MHz, CDCl3) δ: 171.75 (C7), 155.87 (C8), 149.51 (C6),
147.56 (C2), 137.48 (C4), 124.54 (C3), 122.11 (C5), 32.91 (C9), 29.64 (C10-12) ppm. IR (KBr, cm-1):
3444 (w), 3134 (m), 2966 (s), 2834 (m), 1594 (s), 1551 (w), 1443 (s), 1403 (m), 1228 (m), 1124
(w), 1026 (s), 806 (m), 757 (s), 620 (w). LC/MS: m/z 203 [HLt-Bu+H]+. Elemental analysis: Anal.
Calcd for C11H14N4 (Mr=202.25), %: C, 65.32; H, 6.98; N, 27.70. Found, %: C, 65.68; H, 6.75; N,
27.53.
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Typical procedure for preparation of complexes (4a-d). cis-PdCl2(CH3CN)2 (0.26 g, 1 mmol) in
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hot acetonitrile (10 mL) was added to a solution of the proligand (2 mmol) and Et3N (0.29 mL, 2
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DOI: 10.1039/C9NJ02278C
mmol) in acetonitrile (10 mL). Precipitated yellow crystals were collected by filtration, washed with
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acetonitrile and dried in air.
Pd(LEt)2 (4a). Yield: 0.32 g (71%), light yellow crystals. 1H NMR (400 MHz, CDCl3) δ: 10.03 (d,
3JH,H = 5.5 Hz, 1H, Py-H6), 8.05-7.97 (m, 2H, Py-H3,4), 7.41 (t, 3JH,H = 9.2 Hz, 1H, Py-H5), 2.89 (q,
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3JH,H = 7.6 Hz, 2H, H9), 1.42 (t, JH,H = 7.6 Hz, 3H, H10) ppm. 13C NMR (101 MHz, CDCl3) δ:
165.27 (C7), 160.82 (C8), 152.04 (C6), 150.08 (C2), 140.29 (C4), 123.42 (C3), 120.20 (C5), 22.61
(C9), 13.46 (C10) ppm. IR (KBr, cm-1): 3406(w), 2966 (m), 1981 (w), 1615 (s), 1531 (m), 1456 (s),
1306 (m), 1157 (m), 1044 (m), 781 (m),762 (m),706 (m), 444 (w). ESI/MS observed (actual): m/z
453.2 (453.1) [Pd(LEt)2 +H]+. Elemental analysis: Anal. Calcd for C18H18N8Pd (Mr=452,81), %: C,
47.74; H, 4.01; N, 23.50. Found, %: C, 47.77; H, 4.05; N, 23.29.
Pd(Ln-Pr)2 (4b). Yield: 0.36 g (76%), light yellow crystals. 1H NMR (400 MHz, CDCl3) δ: 10.07 (d,
3JH,H = 5.3 Hz, 1H, Py-H6), 8.08-7.96 (m, 2H, Py-H3,4), 7.43 (t, 3JH,H = 6 Hz, 1H, Py-H5), 2.84 (t,
3JH,H = 7 Hz, 2H, H9), 1.87 (sext, 3JH,H = 7.3 Hz, 2H, H10), 1.07 (t, 3JH,H = 7.4 Hz, 3H, H11) ppm. 13C
NMR (101 MHz, CDCl3) δ: 163.41 (C7), 160.15 (C8), 151.58 (C6), 149.47 (C2), 139.74 (C4), 123.01
(C3), 119.70 (C5), 31.34 (C9), 22.57 (C10), 14.37 (C11) ppm.IR (KBr, cm-1): 3022 (w), 2947 (m),
1981 (w), 1616 (s), 1531 (m), 1456 (s), 1091 (w), 790 (m), 753 (m), 434 (w). ESI/MS observed
(actual): m/z 481.2 (481.1) [Pd(Ln-Pr)2+H]+. Elemental analysis: Anal. Calcd for C20H22N8Pd
(Mr=480,86), %: C, 49.95; H, 4.61%; N, 23.30. Found, %: C, 49.83; H, 4.49; N, 23.51.
Pd(Li-Pr)2 (4c). Yield: 0.34 g (74%), light yellow crystals. 1H NMR (400 MHz, CDCl3) δ: 10.12 (d,
3JH,H = 5.0 Hz, 1H, Py-H6), 8.13 (s, 1H, Py-H3), 8.02 (t, JH,H = 7.6 Hz, 1H Py-H4), 7.45 (t, JH,H
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=
6.7 Hz, 1H, Py-H5), 3.26 (spt, 3JH,H = 6.7 Hz, 1H, H9), 1.46 (d, 3JH,H = 6.9 Hz, 6H, H11;10) ppm. 13C
NMR (101 MHz, CDCl3) δ: 169.47 (C7), 160.88 (C8), 152.21 (C6), 150.35 (C2), 140.45 (C4), 123.51
(C3), 120.47 (C5), 29.09 (C9), 22.39 (C11and C10) ppm. IR (KBr, cm-1): 3444 (w), 2966 (m), 1615
(s), 1541 (m), 1447 (s), 1212 (s), 1034 (w), 762 (m), 706 (w), 453 (w). ESI/MS observed (actual):
m/z 481.2 (481.1) [Pd(Li-Pr)2+H]+. Elemental analysis: Anal. Calcd for C20H22N8Pd (Mr=480,86),
%: C, 49.95; H, 4.61; N, 23.30. Found,%: C, 50.03; H, 4.49; N, 23.21%.
1
Pd(Lt-But)2 (4d). Yield: 0.36 g (72%), light yellow crystals. H NMR (400 MHz, CDCl3) δ: 10.16
(d, 3JH,H = 5.4 Hz, 1H, Py-H6), 8.09 (d, 3JH,H = 7.6 Hz, 1H, Py-H3), 8.00 (t, JH,H = 7.5 Hz, 1H, Py-
H4), 7.42 (t, JH,H = 6.4 Hz, 1H, Py-H5), 1.50 (s, 9H) ppm. 13C NMR (101 MHz, CDCl3) δ: 172.06
(C7), 160.82 (C8), 152.06 (C6), 150.41 (C2), 140.24 (C4), 123.30 (C3), 120.51 (C5), 33.38 (C9), 30.13
(C10-12) ppm. IR (KBr, cm-1): 3425 (w), 2947 (m), 1617 (s), 1549 (w), 1456 (s), 1156 (w), 781
(m),743 (m),444 (w). ESI/MS observed (actual): m/z 509.2 (509.1) [Pd(Lt-But)2+H]+. Elemental
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New Journal of Chemistry
analysis: Anal. Calcd for C22H26N8Pd (Mr=508,92), %: C, 51.92; H, 5.15; N ,22.02. Found, %: C,
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51.79; H, 5.29; N, 22.18%.
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DOI: 10.1039/C9NJ02278C
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Physical measurements. The purity of the obtained compounds has been confirmed by IR; 13C, 1H
NMR spectroscopy, elemental analysis and LC\MSD (only proligands). Coordination compounds
were also investigated by UV-vis-fluorescence spectroscopy, MALDI-TOF and ESI mass
spectrometry. Elemental analyses were carried out with Perkin-Elmer 2400 CHN Analyzer.
Melting points (°C, uncorrected) were measured with OptiMelt Automated Melting Point System
(MPA 100). 1H and 13C NMR spectra in CDCl3 solution was recorded on a Varian400 spectrometer
at room temperature. The IR spectra (KBr, pellet) were recorded with Spektrum BX Perkin Elmer
spectrometer. Absorption spectra of complexes in solution were measured by using a UV-vis
spectrophotometer (2800 Unico, USA). The absorbance was registered in cuvettes with l = 1 cm in
the range 200-800 nm using CHCl3 as the reference. The excitation and emission spectra of
palladium complexes in solid state were measured with a LS55 spectrofluorimeter (Perkin-Elmer,
UK) in the optimal conditions at maximum wavelengths for each investigated system. The spectra
were recorded in the range between 200 and 800 nm, emission filter 1%, thickness of the cuvette
was 1 cm, data collection frequency was 0.5 nm, the excitation and emission slit width were 10 and
20 nm, respectively. TG experiments were obtained on Shimadzu DTG-60/60H TGA, with a
heating rate of 10 K min-1. The measurements were carried out in air. The mass spectra of
proligands were recorded on an Agilent 1100 Series high-performance liquid chromatograph
equipped with a diode matrix with an Agilent LC\MSD SL mass selective detector; the ionisation
method is atmospheric-pressure chemical ionisation (APCI). For the MALDI-TOF analysis 1 mg of
studied complex was dissolved in 1 mL of chloroform. Aliquots of mixture were applied to the steel
tips and dried. MS analysis was performed by the method of laser desorption/ionisation on an
Autoflex II (Bruker Daltonics, Germany) mass spectrometer with nitrogen laser (λ = 337 nm).
Experiments were carried out in reflectron mode for positive and negative ions with various time of
delayed extraction (from 1 to 500 ns) in the mass range 50 to 1000 amu. Resulting mass spectra
were obtained by assuming the data of 150 laser shots and processing by the software FlexAnalysis
(Bruker Daltonics, Germany). For ESI MS analysis, 1 mg of the test complex was dissolved in 1 ml
of chloroform. Injection volume 0.5 µL, mobile phase H2O:CH3CN:HCOOH / 1:1:0.05, flow rate
1.5 ml/min. MS analysis was carried out by LCMS analysis using the Agilent G6120A mass
spectrometer (Agilent Technologies, Germany) with electrospray ionisation. The experiments were
carried out in the reflex mode for positive and negative ions in the mass range from 130 to 1000
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amu. Resultant processing of mass spectra was performed by ChemStation software (Agilent
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Technologies, Germany).
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DOI: 10.1039/C9NJ02278C
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X-ray crystallography. Crystals of 4a-d suitable for X-ray analysis were obtained by slow
evaporation of their chloroform solutions. X-ray diffraction data were collected with an Oxford-
Diffraction XCALIBUR E CCD diffractometer equipped with graphite-monochromated MoK
radiation. The unit cell determination and data integration were carried out using the CrysAlis
package of Oxford Diffraction.59 The structures were solved by direct methods using Olex260
software with the SHELXS structure solution program and refined by full-matrix least-squares on
F² with SHELXL-2015.61 Hydrogen atoms attached to carbon were placed in fixed, idealised
positions and refined as rigidly bonded to the corresponding non-hydrogen atoms, except the H
attached to O atom, which were obtained from difference Fourier syntheses and verified by the
hydrogen bond parameters. The molecular plots were obtained using the Olex2 program. The bond
lengths are summarised in Tables S1-S4, while crystallographic data and refinement details are
quoted in Table S5. CCDC – 1910554 (for 4a), CCDC - 1519078 (for 4b), CCDC - 1910553 (for
4c) and CCDC - 1864447 (for 4d) contain the supplementary crystallographic data for this
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contribution.
These
data
can
be
obtained
free
of
charge
via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or deposit@ccdc.ca.ac.uk).
Catalytic studies. The catalytic Suzuki-Myaura cross-coupling reactions were carried out in a
focused Anton Paar Microwave 300 reactor fitted with a rotational system and an IR temperature
detector.
Typical procedure for the catalytic Suzuki-Myaura reaction. 4-Methoxybromobenzene (1.0
mmol), phenylboronic acid (1.2 mmol), potassium or caesium carbonate (1.5 mmol) 4a-d
(1×10−3mmol) and the solvent (3 mL) were mixed in a cylindrical MW Pyrex tube. The tube was
sealed and placed into the MW reactor and stirred (600 rpm) and irradiated (10 W) at 80 °C for the
desired time. Then the reaction mixture was cooled to 25 °C, centrifuged (to separate the catalyst)
and the liquid evaporated to dryness either under a stream of N2 or in vacuo, followed by the
addition of 1,2-dimethoxyethane (1.0 mmol, as the 1H NMR standard) and extraction of the organic
phase with CDCl3. The extract was analysed by 1H NMR spectroscopy. Assignment of the 1H NMR
signals was based on pure samples of substrates and product, while quantifications are performed
upon integration of the selected peak of the product relatively to the peak of the standard. The UV-
vis analyses of the resulting mixture from the C–C coupling reaction were performed in a Lambda
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New Journal of Chemistry
25, Perkin Elmer spectrophotometer, in quartz cells with 1 cm path length. In addition, a mercury
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drop experiment was performed by adding two drops of metallic Hg to the above-mentioned
View Article Online
reaction mixture and running the reaction under the optimised reaction conDdOitIi: o10n.1s0.39/AC9fNteJ0r22t7h8Ce
reaction, the obtained mixture was cooled to room temperature, filtered, and treated and analysed as
above. No significant difference in conversion was observed.
9
The recyclability of 4b was tested by separating the used catalyst from the reaction mixture by
centrifugation followed by filtration of the supernatant solution. Then, the catalyst was washed with
water and methanol and dried in oven at 80 ºC overnight. A new reaction was performed by
addition of new portions (see the quantities specified above) of all reagents besides the catalyst.
After completion of the run, the products were analysed as described above.
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The catalytic assays with conventional heating (oil bath) were carried out in 25 mL round-bottomed
flasks equipped with a reflux condenser using the reaction conditions of the MW experiments
during the desired reaction time.
General procedure for the catalytic Henry reaction. The catalysts 4a-d (5 µmol) were placed,
under air, in a 25 mL round bottom flask. The desired solvent (2 mL), nitroethane (4 mmol) and
benzaldehyde (1 mmol) were added to the flask. The reaction mixture was stirred during 12–24 h,
usually at room temperature and air atmospheric pressure. After the desired time the reaction
mixture was centrifuged to separate the solid catalyst and the solution evaporated to dryness under a
stream of nitrogen. The residue was dissolved in DMSO-d6 and analysed by 1H NMR spectroscopy.
The yield of β-nitroethanol (relatively to benzaldehyde) was established using 1,2-dimethoxyethane
as internal standard, taking into consideration the relative amounts of these compounds, as given by
1H NMR spectra and previously reported.52,62 The values of vicinal coupling constants between the
α-N−C−H and the α-O−C−H protons characteristic of the isomers are J = 7-9 or 3.2-4 Hz for the
syn or anti isomers, respectively. The ratio between the anti and syn isomers was also determined
by 1H NMR spectroscopy. Control experiments were performed under the same reaction conditions
in the presence of PdCl2 instead of 4a-d, and blank experiments (without 4a-d) confirmed that no
products of the Henry reaction were obtained unless the catalyst was added. Recycling experiments
were performed after separating the used catalyst from the reaction mixture by centrifugation
followed by filtration of the supernatant solution. Then, the catalyst was washed with water and
methanol and dried under a stream of nitrogen at ambient temperature. The subsequent cycles were
carried out as above, by adding new portions of all reagents (see the quantities specified above)
except the catalyst. After completion of the run, the products were analysed as described above.
23

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Appendix A. Supplementary Data
† Electronic Supplementary Information (ESI) available: IR, 13C and H NMR spectra, TG-DTA
curves, crystallographic data for compound 4b in cif format, photophysical data,DEOSI: I10a.10n3d9/CM9NAJ0L22D78IC-
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TOF MS. See DOI:
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New Journal of Chemistry
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DOI: 10.1039/C9NJ02278C
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Compound 4a-d revealed good catalytic activity and prospectives of using as mesomorphic materials